If you haven't read it already I thoroughly recommend you read the Q&A with Shari Thurow on the Library & Information Update blog. In the blog post Shari provides excellent definitions of Search Engine Optmization (SEO) and Information Architecture (IA) her definition of SEO is as follows:

SEO is the abbreviation for search engine optimisation (or optimization). On the Web, search engine optimisation is commonly viewed as the process of optimising a website for people who use search engines.

Shari also provides a more detailed definition of IA, which I'm not going to reproduce here as you can read it on full on the blog post or on the Semantic Studios website